Communication devices such as wireless devices are also known as, e.g., User Equipments (UE), mobile terminals, terminals, wireless terminals and/or mobile stations. Terminals are enabled to communicate wirelessly in a cellular communications network or wireless communication system, sometimes also referred to as a cellular radio system or cellular networks. The communication may be performed e.g. between two terminals, between a terminal and a regular telephone and/or between a terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the cellular communications network.
Wireless devices may further be referred to as mobile telephones, cellular telephones, laptops, or surf plates with wireless capability, just to mention some further examples. The terminals in the present context may be, for example, portable, pocket-storable, hand-held, computer-comprised, or vehicle-mounted mobile devices, enabled to communicate voice and/or data, via the RAN, with another entity, such as another terminal or a server.
The cellular communications network covers a geographical area which is divided into cell areas, wherein each cell area being served by an access node such as a base station, e.g. a Radio Base Station (RBS), which sometimes may be referred to as e.g. “eNB”, “eNodeB”, “NodeB”, “B node”, or BTS (Base Transceiver Station), depending on the technology and terminology used. The base stations may be of different classes such as e.g. macro eNodeB, home eNodeB or pico base station, based on transmission power and thereby also cell size. A cell is the geographical area where radio coverage is provided by the base station at a base station site. One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the terminals within range of the base stations. In the context of this disclosure, the expression Downlink (DL) is used for the transmission path from the base station to the mobile station. The expression Uplink (UL) is used for the transmission path in the opposite direction i.e. from the mobile station to the base station.
In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations, which may be referred to as eNodeBs or even eNBs, may be directly connected to one or more core networks.
3GPP LTE radio access standard has been written in order to support high bitrates and low latency both for uplink and downlink traffic. All data transmission is in LTE controlled by the radio base station.
The 3GPP initiative “Licensed Assisted Access” (LAA) intends to allow LTE equipment to also operate in the unlicensed 5 GHz radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes that may be required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum may be shared with other radios of similar or dissimilar wireless technologies, a so called listen-before-talk (LBT) method may need to be applied. Today, the unlicensed 5 GHz spectrum is mainly used by equipment implementing the IEEE 802.11 Wireless Local Area Network (WLAN) standard. This standard is known under its marketing brand “Wi-Fi.”
In Europe, the LBT procedure is under the scope of EN 301.893 regulation. For LAA to operate in the 5 GHz spectrum the LAA LBT procedure may conform to requirements and minimum behaviors set forth in EN 301.893. However, additional system designs and steps may be needed to ensure coexistence of Wi-Fi and LAA with EN 301.893 LBT procedures.
In U.S. Pat. No. 8,774,209 B2, “Apparatus and method for spectrum sharing using listen-before-talk with quiet periods,” LBT is adopted by frame-based OFDM systems to determine whether the channel is free prior to transmission. A maximum transmission duration timer is used to limit the duration of a transmission burst, and is followed by a quiet period.
Long Term Evolution (LTE)
LTE uses OFDM in the downlink and DFT-spread OFDM (also referred to as single-carrier FDMA) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in
FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of SC-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions may be organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in
FIG. 1. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which terminals data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of e.g. the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Rel-11 onwards, above described resource assignments may also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in the above FIG. 3 are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
Physical Downlink Control Channel (PDCCH) and Enhanced PDCCH (EPDCCH)
The PDCCH/EPDCCH is used to carry downlink control information (DCI) such as scheduling decisions and power-control commands. More specifically, the DCI includes:                Downlink scheduling assignments, including PDSCH resource indication, transport format, hybrid-ARQ information, and control information related to spatial multiplexing (if applicable).        A downlink scheduling assignment also includes a command for power control of the PUCCH used for transmission of hybrid-ARQ acknowledgements in response to downlink scheduling assignments. Uplink scheduling grants, including PUSCH resource indication, transport format, and hybrid-ARQ-related information. An uplink scheduling grant also includes a command for power control of the PUSCH.        Power-control commands for a set of terminals as a complement to the commands included in the scheduling assignments/grants.        
One PDCCH/EPDCCH carries one DCI message containing one of the groups of information listed above. As multiple terminals can be scheduled simultaneously, and each terminal can be scheduled on both downlink and uplink simultaneously, there must be a possibility to transmit multiple scheduling messages within each subframe. Each scheduling message is transmitted on separate PDCCH/EPDCCH resources, and consequently there are typically multiple simultaneous PDCCH/EPDCCH transmissions within each subframe in each cell. Furthermore, to support different radio-channel conditions, link adaptation can be used, where the code rate of the PDCCH/EPDCCH is selected by adapting the resource usage for the PDCCH/EPDCCH, to match the radio-channel conditions.
Carrier Aggregation
The LTE Rel-10 standard supports bandwidths larger than 20 MHz. One feature on LTE Rel-10 may be to assure backward compatibility with LTE Rel-8. This may also include spectrum compatibility. That would imply that an LTE Rel-10 carrier, wider than 20 MHz, may appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular for early LTE Rel-10 deployments it may be expected that there may be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. Therefore, it may be necessary to assure an efficient use of a wide carrier also for legacy terminals, i.e. that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. The straightforward way to obtain this may be by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal may receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4. A CA-capable UE is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. A symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. The number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs.
In addition, a key feature of carrier aggregation is the ability to perform cross-carrier scheduling. This mechanism allows a (E)PDCCH on one CC to schedule data transmissions on another CC by means of a 3-bit Carrier Indicator Field (CIF) inserted at the beginning of the (E)PDCCH messages. For data transmissions on a given CC, a UE expects to receive scheduling messages on the (E)PDCCH on just one CC—either the same CC, or a different CC via cross-carrier scheduling; this mapping from (E)PDCCH to PDSCH is also configured semi-statically.
Hybrid Automatic Request Retransmission (HARQ) Protocol
In the LTE system, a user equipment (UE) is notified by the network of downlink data transmission by the physical downlink control channel (PDCCH). Upon reception of a PDCCH in a particular subframe n, a UE is required to decode the corresponding physical downlink share channel (PDSCH) and to send ACK/NAK feedback in a subsequent subframe n+k. This is illustrated in FIG. 5. The ACK/NAK feedback informs the eNodeB whether the corresponding PDSCH was decoded correctly. When the eNodeB detects an ACK feedback, it can proceed to send new data blocks to the UE. When a NAK is detected by the eNodeB, coded bits corresponding to the original data block will be retransmitted. When the retransmission is based on repetition of previously sent coded bits, it is said to be operating in a Chase combining HARQ protocol. When the retransmission contains coded bits unused in previous transmission attempts, it is said to be operating in an incremental redundancy HARQ protocol.
In LTE, the ACK/NAK feedback is sent by the UE using one of the two possible approaches depending on whether the UE is simultaneously transmitting a physical uplink shared channel (PUSCH):                If the UE is not transmitting a PUSCH at the same time, the ACK/NAK feedback is sent via a physical uplink control channel (PUCCH).        If the UE is transmitting a PUSCH simultaneously, the ACK/NAK feedback is sent via the PUSCH.Wireless Local Area Network        
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) may be used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle. When the range of several APs using the same frequency overlap, this means that all transmissions related to one AP might be deferred in case a transmission on the same frequency to or from another AP which is within range may be detected. Effectively, this means that if several APs are within range, they may have to share the channel in time, and the throughput for the individual APs may be severely degraded. A general illustration of the listen before talk (LBT) mechanism in Wi-Fi is shown in FIG. 6.
After a Wi-Fi station A transmits a data frame to a station B, station B may transmit the ACK frame back to station A with a delay of 16 μs. Such an ACK frame is transmitted by station B without performing an LBT operation. To prevent another station interfering with such an ACK frame transmission, a station may be defer for a duration of 34 μs (referred to as DIFS) after the channel is observed to be occupied before assessing again whether the channel is occupied.
Therefore, a station that wishes to transmit may first perform a CCA by sensing the medium for a fixed duration, DIFS. If the medium is idle then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station may wait for the medium to go idle, defer for DIFS, and wait for a further random backoff period.
To further prevent a station from occupying the channel continuously and thereby prevent other stations from accessing the channel, it may be required for a station wishing to transmit again after a transmission is completed to perform a random backoff.
The PIFS may be used to gain priority access to the medium, and may be shorter than the DIFS duration. Among other cases, it may be used by STAs operating under PCF, to transmit Beacon Frames with priority. At the nominal beginning of each Contention-Free Period (CFP), the PC may sense the medium. When the medium is determined to be idle for one PIFS period (generally 25 μs), the PC may transmit a Beacon frame containing the CF Parameter Set element and a delivery traffic indication message element.
Load-Based Clear Channel Assessment in Europe Regulation EN 301.893
For a device not utilizing the Wi-Fi protocol, EN 301.893, v. 1.7.1 provides the following requirements and minimum behavior for the load-based clear channel assessment.
1) Before a transmission or a burst of transmissions on an Operating Channel, the equipment may perform a Clear Channel Assessment (CCA) check using “energy detect”. The equipment may observe the Operating Channel(s) for the duration of the CCA observation time which may be not less than 20 μs. The CCA observation time used by the equipment may be declared by the manufacturer. The Operating Channel may be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in point 5 below. If the equipment finds the channel to be clear, it may transmit immediately (see point 3 below).
2) If the equipment finds an Operating Channel occupied, it may not transmit in that channel. The equipment may perform an Extended CCA check in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total Idle Period that may need to be observed before initiation of the transmission. The value of N may be randomly selected in the range 1 . . . q every time an Extended CCA is required and the value stored in a counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value may be declared by the manufacturer (see clause 5.3.1 q)). The counter may be decremented every time a CCA slot is considered to be “unoccupied”. When the counter reaches zero, the equipment may transmit.
3) The equipment may be allowed to continue Short Control Signaling Transmissions on this channel providing it complies with the requirements in clause 4.9.2.3 in the Wi-Fi protocol, EN 301.893, v. 1.7.1. For equipment having simultaneous transmissions on multiple (adjacent or non-adjacent) operating channels, the equipment may be allowed to continue transmissions on other Operating Channels providing the CCA check did not detect any signals on those channels. The total time that an equipment makes use of an Operating Channel is the Maximum Channel Occupancy Time which may be less than (13/32)×q ms, with q as defined in point 2 above, after which the device may perform the Extended CCA described in point 2 above.
4) The equipment, upon correct reception of a packet which was intended for this equipment, may skip CCA and immediately (see note below) proceed with the transmission of management and control frames (e.g. ACK and Block ACK frames). A consecutive sequence of transmissions by the equipment, without it performing a new CCA, may not exceed the Maximum Channel Occupancy Time as defined in point 3 above. It should be noted that, for the purpose of multi-cast, the ACK transmissions (associated with the same data packet) of the individual devices are allowed to take place in a sequence.
5) The energy detection threshold for the CCA may be proportional to the maximum transmit power (PH) of the transmitter: for a 23 dBm e.i.r.p. transmitter the CCA threshold level (TL) may be equal or lower than −73 dBm/MHz at the input to the receiver (assuming a 0 dBi receive antenna). For other transmit power levels, the CCA threshold level TL may be calculated using the formula: TL=−73 dBm/MHz+23−PH (assuming a 0 dBi receive antenna and PH specified in dBm e.i.r.p.).
An example to illustrate the listen before talk (LBT) in EN 301.893 is provided in FIG. 7.
Truncated Exponential Backoff
In the above basic LBT protocol, when the medium becomes available, multiple Wi-Fi stations may be ready to transmit, which can result in collision. To reduce collisions, stations intending to transmit select a random backoff counter and defer for that number of slot channel idle times. The random backoff counter is selected as a random integer drawn from a uniform distribution over the interval of [0, CW-1]. Note that collisions can still happen even with this random backoff protocol when they are many stations contending for the channel access. Hence, to reduce continuous collisions, the contention window size can be varied.
For the IEEE specs, the default size of the random backoff contention window is set to CWmin. To reduce continuous collisions, the backoff contention window size CW is doubled whenever the station detects a collision of its transmission up to a limit, CWmax, set in the IEEE specs. When a station succeeds in a transmission without collision, it resets its random backoff contention window size back to the default value CWmin.
Licensed Assisted Access (LAA) to Unlicensed Spectrum Using LTE
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that an LTE system may not need to care about coexistence with other non-3GPP radio access technologies in the same spectrum and spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum.
With Licensed-Assisted Access to unlicensed spectrum as shown in FIG. 8, a UE is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application we may denote a secondary cell in unlicensed spectrum as LAA secondary cell (LAA SCell). The LAA SCell may operate in DL-only mode or operate with both UL and DL traffic. Furthermore, in future scenarios the LTE nodes may operate in standalone mode in license-exempt channels without assistance from a licensed cell. Unlicensed spectrum may, by definition, be simultaneously used by multiple different technologies. Therefore, LAA as described above may needs to consider coexistence with other systems such as IEEE 802.11 (Wi-Fi).
To coexist fairly with the Wi-Fi system, transmission on the SCell may conform to LBT protocols in order to avoid collisions and causing severe interference to on-going transmissions. This includes both performing LBT before commencing transmissions, and limiting the maximum duration of a single transmission burst. The maximum transmission burst duration is specified by country and region-specific regulations, for e.g., 4 ms in Japan and 13 ms in Europe according to EN 301.893. An example in the context of LAA is shown in FIG. 9 with different examples for the duration of a transmission burst on the LAA SCell constrained by a maximum allowed transmission duration of 4 ms.
Existing methods for LAA LTE to support LBT in unlicensed spectrum may comprise inappropriate delays of transmission that result in poor performance of a wireless communications network.